I've got a Rottweiler - there are quite a few things that didn't occur to me before I got him or I didn't think would be an issue, Some are not so massive cons, but anyway...
Large breed puppies are prone joint problems, just because of the speed they grow at, you can do everything right and still end up with a dog who can't walk - mine has elbow dysplasia, he's had an operation to try and correct it, he hasn't been walked properly since the beginning of December, he had to have weeks of crate rest and it's cost a fortune.
He's still very much a puppy, so half trained and bouncy - but to people outside he's a big dog jumping at them.
He costs a fortune to keep in appropriate things to chew.
He takes up my full back seat - I can't have him jumping in and out of the boot because he's too young...and now because of his elbow I might never be able to, I can't place him in the boot because he weighs 35kg already.
He can reach my kitchen table without jumping.
When he licks you (and that's a lot) you're blooming soaking because his tongue's huge, lol
It hurts when he stands on your feet and he does it a lot.
He has knocked over toddlers entirely by accident.
He's currently still young enough to want to zoom round the living room, but he's about the size of a full grown lab.
And yep people definitely view him a bit suspiciously, lots don't but enough that you notice it.